Hello. I'm using Logic Express 7 and I have a m-audio mobile preamp. I want to use the m-audio to record on, but want to use the internal sound card for output. How would I configure Logic Express to do this?

This is not possible.
When you set up an audio driver in Logic it is applied for both inputs and outputs.
Some time in the future, Logic may support multiple Core Audio devices, but not yet.

But why would you not want to use the M-Audio output?

And even if Logic does get support for more multiple audio devices, I could imagine it being a feature, whcih would be only for Pro version (not Express - but this is just my speculation).

Actually, it is possible all you need is JACK OSX which is amazing and free!jackosx

a Jack audio connection kit implementation for Mac OS X

When I use the m-audio as the output, all i get is the instrument that is plugged into the m-audio preamp... i don't hear the rest of the song or any effects that i apply to the instrument i'm trying to record. i have garage band set up like this so i figured i could setup logic express the same way.

There should be a mix feature in which you are allowed to mix the amount of signal between what is going into the mbox and what is coming from the software. I know that the Firepod allows this... and many other interfaces.